When opaque substrates are bonded to one another, it is difficult to use usual photocuring adhesives because the substrates do not sufficiently transmit lights. In this case, therefore, heat-bonding is generally adopted. In the heat-bonding, a resin of epoxy type, melamine type, alkyd type, urethane type or acrylate type and a heat-curing agent are used in many cases and the use of a hot-melt type resin or the like is also known.
However, in the heat-bonding, there have been such problems that the substrates are warped or distorted owing to heat, and hence, the kind of substrate to which the heat-bonding can be applied has been limited. Moreover, the hot-melt type resin is inferior in thermal stability and weather resistance, and has been difficult to use under high temperature circumstances. In addition, the heat-bonding has required a curing time of several tens of minutes and hence has had a problem that it is inferior in productivity.
Therefore, there has been desired a development of an adhesive and a bonding method which are free from such problems as warpage and distortion of substrate, productivity and the like. In particular, in order to bond optical discs which will be required to have a higher recording density in future, there must be used an adhesive which causes neither warpage nor distortion of disc and is endowed with excellent characteristics.